The present invention relates to a method and apparatus for calibrating a light emission measurement heating chamber to automatically compensate for varying backside conditions on a semiconductor wafer.
Recently, there has been a trend toward single wafer processing of semiconductor wafers, rather than batch processing. Single wafer processing ensures that the processing of each wafer is more precise and that there is a higher level of consistency between wafers. Therefore, when the wafer is transformed into its final form, it functions more predictably and more accurately. As increasing densities of components on wafers are developed, the need for very precise processing increases. In order to achieve throughputs similar to conventional wafer ovens which process a large number of wafers at once, a rapidly heating arc lamp is used, instead of a heating element, to speed up the processing time.
Apparatus for rapid thermal processing of semiconductor wafers are known in the prior art. For example, U.S. Pat. No. 4,755,654 (hereinafter '654) discloses a semiconductor wafer heating chamber for applying either a desired uniform or non-uniform heating pattern to a wafer. The apparatus of '654 includes a long-arc AC gas-discharge lamp with a spectral output tuned for absorption by silicon. The lamp is capable of quickly raising the temperature of the wafer to a desired process temperature and holding it there for the time period necessary to accomplish the specific step of the process. Since the lamp heats the wafer quickly, the apparatus of '654 can be used efficiently to process wafers one at a time. The lamp heats the wafer with light directed to the top, or front side, of the wafer. A pyrometer is used to detect the infrared light emitted from the backside of the wafer when it is heated. This light is proportional to the temperature of the wafer. The pyrometer output is provided as a feedback to the heating system.
A pyrometer works by measuring the amount of radiation emitted in a certain spectral band (or bands) from the object to be measured. All objects emit radiation if they are at any temperature above absolute zero. The emitted radiation can be described quantitatively in a very simple form by the Stefan-Boltzmann Law. The radiant energy equals the emissivity times the Stefan Boltzmann constant times the temperature to the fourth power.
The spectral content of this radiation can also be determined using Plank's Law. Since one can calculate the amount of radiation emitted and the spectral distribution of the radiation, it should be simple to measure the radiation and work backwards to calculate the temperature. This would be true except that the factor "emissivity" is not, in most cases, a known constant. In fact, it is usually not a constant at all, but a function of wavelength and temperature. Therefore, to use pyrometry to accurately measure temperature, one must calibrate the system by effectively measuring "emissivity".
The calibration of the pyrometer feedback value indicating the temperature of the heating chamber is critical in the processing of a particular wafer. Presently, calibration of pyrometers used in wafer processing ovens is accomplished manually. The pyrometer is exposed to a light of a known value and the gain of the pyrometer output is adjusted to the desired value for such value of light using a potentiometer.